Process for surface modification of polyethylene terephthalate film

ABSTRACT

An improved process for surface modification of polyethylene terephthalate film is provided, involving corona discharge treatment of the film in an atmosphere of nitrogen containing up to about 350 ppm oxygen, and optionally a small amount of acetone vapor. Films so treated exhibit improved surface properties.

BACKGROUND OF THE INVENTION

This invention relates to an improved process for surface modificationof polyethylene terephthalate (PET) film involving corona discharge.

The treatment of films or other shaped articles made of PET using acorona discharge to improve their surface characteristics such asadhesive properties is well known. However, such treatment is ofteninsufficient to achieve the desired improvements. Several attempts havetherefore been made to improve the surface characteristics beyond thoseobtainable by simple corona discharge treatment.

U.S. Pat. No. 4,594,262, to Kreil et al., discloses a method for coatinga polyester film base with a curable organic coating, by (1)continuously passing uncoated polyester film base through an inertatmosphere containing less than 100 ppm oxygen, while (2) exposing thefilm base to irradiation by an electron beam, to subject the film baseto an absorbed dosage of at least 1 Mrad, and (3) applying a curableorganic coating to the surface of the polyester film. The polyesterfilm, after step (2), is substantially more adherent to organic coatingsthan such film which has not been so treated, and this greater adherencelasts for prolonged periods during storage at room temperature in air.

French patent No. 2,544,324 discloses a process for increasing theadherence of the surface of a shaped product made of a polyestercomposition. This process consists of the application of a coronadischarge to this surface, which is continuously moving between adischarge electrode and another, opposite electrode. An atmospherecontaining oxygen in an amount not exceeding 20% by volume is projectedonto the surface to which the corona discharge is applied.

SUMMARY OF THE INVENTION

This invention provides a process for treating a polyester film with acorona discharge in an apparatus comprising at least one electrode and ahousing provided with a means for admitting a gas and substantiallyexcluding ambient air, comprising the steps of:

(a) providing an atmosphere, within the housing, of nitrogen containingup to about 350 parts per million molecular oxygen;

(b) adjusting the energy density of the corona discharge to a value ofabout 90 to about 140 watts per square inch;

(c) continuously passing said film through said housing; and

(d) exposing said film, within said housing provided with said nitrogenatmosphere, to said corona discharge, wherein the exposure of said filmto said corona discharge is from about 2 to about 64 watt minutes ofcorona energy per square foot of film.

The present invention also provides film prepared by the above process.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process for treating a film or othershaped article of PET with a corona discharge under an atmosphere ofnitrogen, in order to improve the adhesion characteristics of the film,by increasing the total adhesion and reducing the variability in theadhesion, when compared to corona treatment in an atmosphere of air.

The film which may be successfully treated using this process includespolyester film. Various types of polyester film may be used, but thepreferred polyester is polyethylene terephthalate.

In order to attain the improvements of the present invention, it isnecessary that the atmosphere overlaying the PET film in the regionwhere the corona discharge treatment is being effected is replaced by anatmosphere containing nitrogen, from which oxygen has been substantiallyremoved. An atmosphere of nitrogen containing no more than about 320parts per million oxygen has been found to be suitable. Other materialsmay also be present in the nitrogen. For example, we have found thatsuccessful results may be obtained at low oxygen levels when acetonevapor is also present in the nitrogen, at levels up to saturation. Butit is preferred that nitrogen, without any other added materials, beused.

Devices suitable for corona treatment of films are well known in theart, and are described, for example, in U.S. Pat. No. 3,133,193.Generally, a suitable configuration includes a grounded metal roll withan insulated cover, and an electrode mounted parallel to the cylinderaxis of the roll. The film passes over the insulated roll, and thecorona is developed between the electrode and the film. The electrodegap, which is the distance between the electrode and the insulated rollcover should be about 30 to 100 mils, preferably about 40 mils. Thecorona discharge apparatus of this invention also includes a means forsupplying nitrogen, and a means for maintaining the nitrogen atmosphere,excluding ambient air. Such a modified apparatus is also well known, andis illustrated, for example, in French Pat. Appl. No. 2,544,324.

The amount of energy applied to the film is important, in that a certainminimum amount is needed in order to achieve the desired improvements insurface properties of the film, and an upper limit is normally set byeconomic considerations. The energy applied is expressed as the powerconstant, which is traditionally in units of watt-minutes per squarefoot of film. The power constant is equal to the corona power, in watts,divided by the product of the film width and the line speed, in feet perminute. Suitable values for the power constant are about 2 to roughly 64W-min/ft2 The upper limit is quite flexible. For economic reasons, apower constant of about 3 to about 10 are preferred, and most preferablynear 3. At power constants much below 3, the benefits of this inventionare less fully realized. A variety of apparatus and geometricalarrangement are possible to achieve this level of treatment. Some suchparameters are further described in the following paragraphs.

The corona power required will vary with the size of the apparatus andthe rate of treatment of film. A small, laboratory scale corona treatermay be designed to treat a web of film 4 inches wide, at a rate of about7 to 72 feet per minute. Such an apparatus may use a discharge of about50 to about 150 watts in order to obtain a suitable power constant.Larger commercial corona treaters require correspondingly more power.For example, treating a 6 foot wide film web at a line speed of 500 or1000 feet per minute, the required corona power is 9 or 18 kW,respectively, to obtain a power constant of 3 W-min/ft². The frequencymay be typically about 18-40 kHz. A laboratory scale treater may have anelectrode area of about 0.36 to about 2.25 square inches, so that theelectrode energy density is typically in the range of about 44 to about150 watts/in² A commercial unit would have a correspondingly largerelectrode area.

We have found that the improvement of this invention can be achievedusing a variety of combinations of electrode energy density and powerconstant. These two factors are interrelated. At power constants muchbelow 3 watt min/sq. ft., and at electrode energy densities much belowabout 100 watts/sq. in., the absolute values of adhesion diminish,although the samples corona treated in a nitrogen atmosphere maycontinue to be superior to those corona treated in air. Thus thisinvention is particularly useful when the electrode energy density is atleast about 90, or preferably at least about 100 watts/sq. in., and thepower constant is at least about 2, or preferably at least about 3 wattminutes/square foot. Film width, speed, and other parameters can beadjusted by the person skilled in the art to achieve these values.

The improvements obtained by treating films of PET according to thisinvention are illustrated in the following nonlimiting examples:EXAMPLES Example 1

A laboratory scale corona treater is located in a scrap vacuumevaporator. The corona treater includes a 3.5 inch diameter groundedcorona roll with a fluoropolymer cover. A 4 inch long electrode, havingan area of 2.25 square inches, is mounted parallel to the cylinder axisof the roll, to provide an electrode gap of 40 mils. The apparatus isprovided with a controlled nitrogen atmosphere by evacuating to about 28inches of Hg vacuum, then returning the unit to atmospheric pressure byback filling with nitrogen. This procedure is repeated two more times.After the third filling, the unit is purged with nitrogen until anoxygen concentration of less than 10 ppm is reached, as measured using a"Teledyne" oxygen meter. (During the treatment the oxygen concentrationdrops to 3.5 ppm.) A corona is generated by supplying 150 watts at 22.5kHz to the electrode from a solid-state corona power supply. A sample of0.5 mil PET film is passed through the corona using a D. C. poweredmotor, at 7 ft/min, as measured by a tachometer. These conditionscorrespond to a power constant of 64 watt min/sq. ft. and an electrodeenergy density of about 66 watts/sq. in.

After treatment, the film is removed from the apparatus and coated withindia ink by spreading the ink across the film with a handproofer untilthe film is covered. The ink, after drying, cannot be removed using"Scotch" brand 810 adhesive tape. This test indicates satisfactoryadhesion.

Example 2

A series of experiments is run similar to that of Example 1. The linespeed of the corona treater is 32.5 ft/min. The 0.5 mil PET film iscorona treated in an air atmosphere, a nitrogen atmosphere, or anitrogen atmosphere saturated with acetone (designated as"Air,""Nitrogen," and "Acetone" in the Tables below). The oxygenconcentration in the treater is 4.5 ppm when treating with nitrogen and3.4 ppm when treating with acetone/nitrogen. Using an electrode area of1.06 sq. in., 100 watts of corona energy at 21.4 kHz is applied duringtreatment. These conditions give a power constant of 9.2 watt min/sq.ft. and an electrode energy density of 94 watts/sq. in.

The film is evaluated by an ink adhesion test similar to that inExample 1. In this test, an amount of ink is applied which is sufficientto cover the film surface. Two types of ink are typically used,Converters Ink Co. Ink #P-73741 ("white") and Ink #P-73745 ("red").These inks are diluted with 90% ethanol/10% propyl acetate to aviscosity of about 22 seconds as measured in a Zahn viscosimeter #2 cup.The ink sample is drawn across the film surface using a "Parmarco HandProofer" with a #165 steel roll, before significant solvent loss due toevaporation can occur. The inked film is dried in an oven at about 85 Cfor about 10 seconds, cooled, and allowed to age for at least 5 minutes.A 6 inch minimum length of adhesive tape ("Scotch"#610 or equivalent) isapplied to the ink and pressed down with a roller. The tape is thenpeeled back at an angle of a little less than 180°. A slow steady pullis used for the first half of the contact area ("Slow"), followed by arapid, "snapping" removal rate for the remainder ("Fast"). The resultsare reported numerically as follows:

10: no ink removed by the tape

9: 90% of ink remaining, 10% removed etc.

The results of the ink adhesion test are shown in Table I, and arecompared with a commercial grade of PET film which is corona treated inair, a product from E. I. du Pont de Nemours and Co, Inc.

                  TABLE I                                                         ______________________________________                                        Ink: White Red Red on White                                                    Rate: slow/fast slow/fast slow/fast                                          System:                                                                       ______________________________________                                        Nitrogen    9/9          9.5/9  10/10                                         Acetone     8.6/5.3      7/6    10/9                                          Air         6/5          8/6.5  3/3                                           Commercial  8/5          --     4/5                                           ______________________________________                                    

Example 3

Example 2 is repeated, but at a line speed of 50 ft/min. Theseconditions give a power constant of 6.0 watt min/sq. ft. and anelectrode energy density of 94 watts/sq. in. The results are shown inTable II, wherein the heading are the same as in Table I.

                  TABLE II                                                        ______________________________________                                        Nitrogen  10/10        10/l0   l0/10                                          Acetone   10/9         10/10   10/9                                           Air       5/7.7        10/10   4/9                                            ______________________________________                                    

Example 4

PET film is treated as in Example 2, at a line speed of 71-73 ft/min.The power constant is thus somewhat lower than in Examples 2 and 3,being 4.2 watt min/sq. ft. Adhesion values are shown in Table III,wherein the headings are the same as in Table I. This increase in linespeed results in a decrease in the adhesion values.

                  TABLE III                                                       ______________________________________                                        Nitrogen   10/2         9/5    9/2                                            Acetone    10/2         0/0    10/6                                           Air        6/7          10/0   2/6.5                                          ______________________________________                                    

Example 5

PET film is treated as in Example 4, with a line speed of 71-73 ft/min,an electrode area of 0.531 sq. in., and 75 watts electrode power. Theseconditions give an electrode energy density of 140 watts/sq. in. and apower constant of 3.1 watt min/sq. ft. Duplicate measurements are made.A marked increase in adhesion is obtained for nitrogen and air treatedfilm when compared with values of Example 4. The increase adhesion ofthis example is obtained at a higher electrode energy but a lower powerconstant than Example 4. The results are shown in the Table IV.

                  TABLE IV                                                        ______________________________________                                        Nitrogen  10/10        10/10   10/10                                          Acetone   0/0          10/9.7  7.3/8.7                                        Air       10/9.7       10/10   9/9.7                                          ______________________________________                                    

Example 6

PET film is corona treated in the presence of nitrogen under theconditions of Example 5. However, the oxygen content is varied as shownin Table V. Using these conditions, excellent adhesion results usingoxygen concentrations of up to 350 ppm.

                  TABLE V                                                         ______________________________________                                         5 ppm O.sub.2                                                                           10/10        10/10   10/10                                         175 ppm    10/10        10/10   10/10                                         350 ppm    10/10        10/10   10/10                                         ______________________________________                                    

Example 7

PET film is corona treated as in Example 5 except the electrode energydensity is 125 watts/sq. in. Adhesion values are shown in Table VI.

                  TABLE VI                                                        ______________________________________                                        Nitrogen  10/10        10/10   10/9.5                                         Acetone   0/0          10/10   0/3.5                                          Air       1/8          10/10   3/8                                            ______________________________________                                    

Example 8

Pet film is corona treated as in Example 5 except the electrode energydensity is 110 watts/sq. in. Adhesion values are shown in table VII.

                  TABLE VII                                                       ______________________________________                                        Nitrogen  9.5/10       10/10   9/10                                           Acetone   0.5/0.5      10/10   0/3                                            Air       6.5/5        10/10   5.5/10                                         ______________________________________                                    

Example 9

Pet film is corona treated as in Example 5 except the electrode energydensity is 100 watts/sq. in., the total electrode energy is 50 watts,and the line speed is 48.5 ft/min. The power constant is 3.1 wattmin/sq. ft. as in Example 5. Table VIII shows that the results aremarginal for corona treatment in a nitrogen atmosphere under theseconditions.

                  TABLE VIII                                                      ______________________________________                                        Nitrogen  9.2/7.4      9.4/9.6 6.2/10                                         Acetone   0/0          10/10   0/5.2                                          Air       6.2/8.2      10/10   2.8/8.6                                        ______________________________________                                    

Example 10

Pet film is corona treated as in Example 5, with an energy density of140 watts/sq. in., except the power constant is 2.1 watt/sq. ft. and theelectrode energy is 50 watts. The results in Table IX indicate that theadhesion properties are not as good as those obtained at higher powerconstants, but the samples run under nitrogen continue to give improvedresults compared to air.

We claim:
 1. A process for treating a polyester film with a coronadischarge in an apparatus comprising at least one electrode and ahousing provided with a means for admitting a gas and substantiallyexcluding ambient air, comprising the steps of:(a) providing anatmosphere, within the housing, of nitrogen containing up to about 350parts per million molecular oxygen; (b) adjusting the energy density ofthe corona discharge to a value of about 90 to about 140 watts persquare inch; (c) continuously passing said film through said housing;and (d) exposing said film, within said housing provided with saidnitrogen atmosphere, to said corona discharge, wherein the exposure ofsaid film to said corona discharge is from about 2 to about 64 wattminutes of corona energy per square foot of film.
 2. The process ofclaim 1 wherein the atmosphere of nitrogen contains up to about 175parts per million oxygen.
 3. The process of claim 2 wherein theatmosphere of nitrogen contains up to about 5 parts per million oxygen.4. The process of claim 3 wherein the film is exposed to about 3 toabout 10 watts minutes of corona discharge per square foot of film. 5.The process of claim 4 wherein the energy density of the coronadischarge is about 100 to about 140 watts per square inch.
 6. Theprocess of claim 5 wherein the width of said film is about 6 feet andthe speed of said film through said housing is about 500 to about 1200feet per minute.
 7. The process of claim 5 wherein the film ispolyethylene terephthalate.
 8. The process of claim claim 3 wherein thenitrogen contains acetone vapor.
 9. A film prepared by the process ofclaim
 1. 10. A film prepared by the process of claim
 4. 11. A filmprepared by the process of claim
 6. 12. A film prepared by the processof claim 7.